Sometimes the area of the tube that's beyond the patch that's been softened by the vulcanizing fluid will stick to the tyre. There's some rubbing between the tube and tyre, so some talc will help prevent the repair from sticking to the inside of the tyre. The rubbing will move the sticking part differently than non-sticking and can loosen a patch before it's fully vulcanized to the tube.
Personally I've had good luck with having some patience and clamping the patch & tube between two blocks of wood in a vise overnight.
To those who are mechanically inclined and have some precision in their thinking and work, you can compare a "glue" versus "vulcanizing" fluid as the difference between brazing and welding. Or do a test, lay down a thin stripe of "glue", "contact cement" or "rubber cement" on one section of tube. And near it, lay down a stripe of "vulcanizing fluid". Walk away for 5-10 minutes and come back and rub on the two spots. The area with "glue", "contact cement" or "rubber cement" will rub off in little rubber balls (you've all done this with rubber-cement on paper before). However, the spot with true "vulcanizing fluid" will have nothing come off.
Anyone ever build model planes, cars, ships when you were kids? Remember the difference between the thick gooey glue versus the thinner solvent "glues"?